Method for removing weakly acidic sulphur compounds from hydrocarbon oil



' acids.

Patented Aug. 13, 1946 METHOD FOR REMOVING WEAKLY ACIDIC SULPHUR COMPOUNDS FROM HYDRO- CARBON OIL George W. Ayers, Chicago, and Daniel M. Barton, Elmwood Park, 111., assignors to The Pure Oil Company, Chicago, Ill., a corporation of Ohio No Drawing. Application June 21, 1943, Serial No. 491,668

8 Claims.

This invention relates to a method and reagent for removing weakly acidic bodies from waterimmiscible organic fluids and is more particularly concerned. with a method and reagent for removing mercaptans from petroleum oils,

In Patent No, 2,316,966 there is disclosed and claimed a method for removing acidic bodies from water-immiscible fluids by contacting such fluids with aqueous alkali solutions containing the reaction product of an alkali and high-boiling tar We have found that high-boiling acids may cause difficulties when used in the extraction of mercaptans from gasoline and other petroleum distillates. These tar acids contain substances which impart to the gasoline or other oil which is treated therewith, objectionable color and cloudiness. Moreover, in certain instances, alkali solutions containing these tar acids become very viscous and even solidify at temperatures of 40 F. or lower, with the result that emulsion difficulties are encountered during use of such solutions. Gasoline treated with alkali solutions containing high-boiling tar acids may acquire an unpleasant odor. A further difliculty which is sometimes experienced is the interference with the analytical determination of the mercaptan sulphur content of the treated gasoline. Difiiculty is experienced in determining the end point during the titration of the gasoline using the silver nitrate-ammonium thiocyanate titration method disclosed in the Ma 13, 1941, analytical edition of Industrial and Engineering Chemistry, volume 13, page 298 in an article by Bell and Agruss entitled, Analytical procedure for mixtures of organic sulphur compounds.

We have discovered that all of the foregoing diihculties can be eliminated if the high-boiling tar acids are treated to eliminate therefrom, those constituents soluble in water which impart to the aqueous solution a greenish-black color in the presence of substantially acid-free ferric chloride. It is not certain whether or not the substances which cause the dimculty are those substances which turn aqueous solutions a greenish-black upon addition of ferric chloride, but if those substances which do turn the solution a greenishblack are removed from the tar acids, the remaining tar acids cause no difiiculty when used in alkali solution to treat petroleum oils.

An object of the invention is to provide an improved method for removing weakly acidic bodies from otherwise neutral organic water-immiscible fluids.

Another object of the invention is to provide a method for removing mercaptans from petroleum distillates.

A further object of the invention is to provide a method for removing from high-boiling tar acids, constituents which are objectionable when tar acids are used as solubility promoters in alkali solutions to remove weakly acidic bodies such as mercaptans from hydrocarbon or other substantially neutral water-immisible fluids.

A still further object of the invention is to provide an improved solubility promoter for use in extracting weakly acidic bodies from organic water-immiscible fluids.

Other objects of the invention will appear from the following description:

In the low-temperature, by-product, destructive distillation of coal, there is ordinarily contained a dark liquid called coal tar which upon distilaltion produces oils of different boiling ranges and a residual pitch. It is common practice in the coal-tar industry to distill coal-tar into several fractions; one being light oil, boiling up to about 200 6., another known as middle oil boiling from about 200 to 250 0., a third fraction known as heavy oil boiling from about 250 to 300 C., and a fourth fraction known as anthracene oil boiling from about 300 to 350 C. It is common practice to treat fraction with aqueous alkali solutions to remove therefrom the acidic substances contained therein. The resulting aqueous alkali solution is neutralized with acid to recover the acidic substances extracted from the various oil fractions. The acidic substances are then fractionated to obtain various phenols such as, phenol, cresols and one or more xylenols. In the fractionation, the separation is made between those constituents boiling below and above approximately 220 C. The residual liquid from the fractionation of the tar acids is the material A solution was prepared, using high-boiling tar acids made in the manner just set forth and obtained from the Reilly Tar & Chemical Corporation by mixing together 20% dry sodium hydroxide, 20% tar acids and 60% of water, by weight. The resulting solution was not entirely homogeneous, but contained liquid or solid material which had a shiny appearance. 300 cc. of Dubbs cracked gasoline from Texas crude oil was agitated with 7% by volume of the'solution and the resulting gasoline Was colored yellow-brown. The treating solution continued to impart the yellow-brown color to successive batches of gasoline. In addition, the gasoline was cloudy. An attempt was made to determine the mercaptan sulphur content by the silver nitrate-ammonium thiocyanate titration method, but considerable difficulty was experienced in determining the end point of the titration. Moreover, emulsion difliculties were encountered when an attempt was made to counter-currently contact gasoline with the treating solution in a counter-current contact tower.

Another treating solution was prepared by mixing together 20% of NaOH, 20% of water-washed, high-boiling tar acids and 60% of water by weight. The water washing of the high-boiling tar acids was carried out by vigorously agitating high-boilin tar acids with an equal volume of boiling water for one-half hour and drawing oif the upper aqueous layer and repeating the washing step with an equal volume of fresh boiling water three additional times. Each aqueous layer was tested by adding thereto a few drops of freshly prepared aqueous ferric chloride solution containing approximately 2% of'ferric chloride. The aqueous layer from the first two washings gave a greenish-black color upon addition of the ferric chloride while the aqueous layer from the last two washes gave a violet-black color with the addition of the ferric chloride. The concentration or amount of ferric chloride solution used in the test is not important since the color reaction occurs in the presence of a very small amount of the ferric chloride. It is important that the ferric chloride be substantially acidfree since acid interferes with the color change. Therefore, freshly prepared ferric chloride solution should be used.

The alkali solution prepared by using the water-washed tar acids wa homogeneous even when cooled to 40 F. A sample of the same Dubbs cracked gasoline from Texas crude was treated with 7% by volume of the aforesaid alkali solution containing the washed high-boiling tar acids. The treated gasoline was only slightly colored, contained no appreciable'haze or cloudiness, had no objectionable foreign odor, caused no troublesome emulsoin in the treating step and could be titrated for mercaptan sulphur with ease. There was no difference in the amount of mercaptan sulphur removed by the solution containing the unwashed and the washed high-boiling tar acids.

The mercaptan sulphur content of the gasoline was lowered from 0.096% to 0.0231% in a single treatment with 7% by volume of the treating solution. A treating solution containing 20% by volume of solid sodium hydroxide, 20% of waterwashe d tar acids, 5% of commercial cresol and 55 of water by weight, when contacted with the same gasoline in an amount of 7% by volume, lowered the mercaptan sulphur content to 0.01'76% and caused no objectionable odor, color, haze or emulsion.

A sample of the same gasoline was treated with 7.6% by volume of the solution containing 20% by weight of soduim hydroxide, 20% by weight of water-washed high-boiling tar acids and 60% by weight of water in a counter-current tower. The mercaptan sulphur of the gasoline was lowered from 0.096% to 0.005. A comparison of the octane numbers and lead susceptibilities of the gasoline before and after treatment is given in the These figures show a notable increase in lead susceptibility of the treated over the untreated gasoline. In order to make a '72 octane gasoline from the treated sample, only 0.6 cc. of tetraethyl lead was required, thereby effecting a saving of 40% in the quantity of tetraethyl lead over that necessary to bring the'untreated sample up to 72 octane.

, It should be understood that water-washing of the high-boiling tar acids need not be done with boiling water, nor i it necessary to use the ratio of acids to water or the particular number of washes herein specified. Water of lower temperature may be used, but will require a larger amount of washing or a longer washing period to remove the objectionable substances from the tar acid. Larger ratios of water to tar acids will reduce the number of washings. We prefer water approaching the boiling point in temperature because of the viscous character of the highboiling tar acids. If it is desired to use cold water or water which is only slightly warm, it may be expedient to dilute the tar acids with an inert water-immiscible fluid to lower the Viscosity, and to remove the diluent before adding the tar acids to the alkali solution. 1

The extent of water-washing in any case, is determined by testing the wash water with ferric chloride solution. When the color of the washed Water upon addition of ferric chloride, changes from greenish-black to violet-black, the washing is sufficient.

Our invention is not limited to the particular manner for removing the objectionable substances from high-boiling tar acids, but it is intended to include the us of high-boiling tar acids as solubility promoters where tar acids are free of substances which dissolve in Water to give a greenish-black color on addition to the water of ferric chloride.

Although for purposes of demonstrating the invention we used a solution made by mixing together 20% of sodium hydroxide, 20% of waterwashed high-boiling tar acids and water, the composition of the solution may vary within wide limits. In order to obtain appreciable removal of mercaptans from hydrocarbon oils the treating solution should contain at least 5% by weight of free alkali metal hydroxide, i. e. alkali metal hydroxide over and above that necessary to react with other constituents of the solution, and at least 5% by weight of high-boiling tar acids, but preferably in excess of 10%. Either sodium or potassium hydroxide or a mixture thereof may be used as the alkali metal hydroxide. Care should be exercised to keep the free alkali hydroxide concentration of the solution below the point at which the solution becomes so viscous as to cause emulsion difficulties during the treating steps. Free sodium hydroxide concentrations up to 20% by weight and free potassium hydroxide concentrations up to 25% by weight are satisfactory. The washed tar acids may be added to the solution as such or in the form of alkali metal salts. The tar acid content of the solution may extend up to the maximum amount which is compatible with the solution and will vary with the alkali concentration of the solution. Other solubility promoters such as phenols, low boiling fatty acids and naphthenic acids may be used as components of the treating'solution.

Treatment of gasoline and other hydrocarbon distillates by means af alkali solutions containing tar acids prepared in accordance with our invention may take place at ordinary atmospheric temperature and preferably within the range of approximately 40-100 F. The ratio of treating solution to oil undergoing treatment will depend on the nature and amount of acidic bodies contained in the oils, but in general, a ratio of 5 to 20% of treating solution by volume based on the oil undergoing treatment can be used with good results. The treating solution can be regenerated by boiling or by stripping the absorbed acidic constituents therefrom by means of steam at temperatures of approximately 220-250" F.

It is claimed:

1. The method of removing weakly acidic sulphur compounds from hydrocarbon oil which comprises contacting said oil with aqueous solution containing at least 5% by weight of free alkali metal hydroxide and a sufiicient amount of the soluble reaction products of an alkali and high-boiling tar acids which have been freed by water washing of constituents which react with ferric chloride in aqueous solution to impart thereto a greenish-black coloration to enhance the ability of said solution to extract said sulphur compounds from the hydrocarbon oil.

2. The method of removing weakly acidic sulphur compounds from hydrocarbon oil which comprises contacting said oil with aqueous solution containing at least 5% by weight of free alkali metal hydroxide and a sufiicient amount of the soluble reaction products of an alkali and higher boiling tar acids which have been washed sufficiently with water to remove therefrom those constituents which in aqueous solution turn greenish-black in the presence of ferric chloride to enhance the ability of said solution to extract said sulphur compounds from the hydrocarbon oil.

3. Method in accordance with claim 1 in which the aqueous solution contains at least 10% by weight of high-boiling tar acids in the form of their reaction products with alkali.

4. The method of removing mercaptans from gasoline which comprises contacting the gasoline with an aqueous solution made by mixing together approximately 20 parts by weight of alkali metal hydroxide, 20 parts by weight of highboiling tar acids which have been water washed to remove therefrom objectionable gasoline-soluble color-impartingbo'dies and sufficient water to keep the alkali metal hydroxide and tar acids in solution.

5. The method of improving high-boiling tar acids for use as solubility promoters in alkali solutions which consists in washing said tar acids with at least two volumes of water until the wash water changes from greenish-black to violet-black color upon addition of ferric chloride.

6. The method of improving high-boiling tar acids, for use as solubility promoters in alkali solutions which consists in washing said acids with at least two volumes of hot water until the wash water no longer turns greenish-black upon additionof ferric chloride.

7. The method of removing from high-boiling tar acids substances which discolor gasoline when contacted with alkali solutions containing said tar acids which consists in washing said tar acids with at least four volumes of boiling water, the amount of water being suflicient to remove from said tar acids substances which turn water containing them greenish-black upon addition of ferric chloride.

8. Method in accordance with claim 1 in which the aqueous solution contains not less than 10% by weight of free alkali metal hydroxide but less than that amount which causes the solution to be so viscous as to cause emulsion difficulties when mixed with the hydrocarbon oil, and in ex-- cess of 10% by weight of high boiling tar acids in the form of reaction products with the alkali metal hydroxide.

GEORGE W. AYERS. DANIEL M. BARTON. 

